Propagation of repetitive alpha waves over the scalp in relation to subjective preferences for a flickering light
Introduction
The efforts to describe important qualities of sound and light in terms of the processes of the auditory and visual pathways and brain has been brought to bear on the problem (Ando and Noson, 1997). If enough were known about how the auditory, the visual, and the central nervous systems modify the nerve impulses from the cochlea and retina, the design of concert halls and lighting, for example, could proceed according to guidelines derived from knowledge of these processes. Here, attempts to investigate for this purpose have been made through studies of electroencephalogram (EEG).
Techniques that are based on the autocorrelation function (ACF) and the cross-correlation function (CCF) have been developed to describe the nature of EEGs (Braizer and Casby, 1952, Barlow, 1961, McLachlan and Shaw, 1965, Liske et al., 1967, Hoovey et al., 1972). A technique based on the CCF has been developed for identifying, in humans, dynamic spatiotemporal electrical patterns of the brain during purposive behaviors (Gevins et al., 1981, Gevins et al., 1983). The interhemispheric relationships during sleep have been investigated by using the CCF (Barcaro et al., 1986). The persistence of oscillations in the ACF has been used for identifying seizure activity and for being of clinical utility for characterizing electroconvulsive therapy seizures (Krystal et al., 1996). The ACF is used to determine characteristics over time, that is, the degree of persistence of a signal. The CCF is used to examine mutual relationships for signals detected at two electrode sites, the presence of common components, and transmission times.
The oscillatory components of EEG have been related to perception and cognitive processing (Singer and Gray, 1995). Visual stimulation and cognitive activity suppress alpha waves while increasing the power of the high frequency beta and gamma bands (Pfurtscheller and Klimesch, 1990). Temporal spectral evolution analysis showed that the decrease in alpha band and the increase in beta band during visual spatial attention (Marrufo et al., 2001). Alpha waves, which have the longest period of the EEG forms seen in subjects who are awake, are also thought to indicate pleasant and comfortable feelings. The differentiation of basic emotions, i.e. intention, anxiety, aggression, sadness, and joy by means of EEG-coherences has been discussed extensively (Hinrichs and Machleidt, 1992). Intention, aggression and joy are mainly characterized by an increase in alpha-coherence, whereas a decrease is seen for anxiety and sorrow. Changes in the EEG that appeared with mental processes of a higher order have been analyzed by using coherence analysis (Petsche, 1996). Acts of creative visual thinking were characterized by increased alpha-coherence between occipital and frontopolar electrode sites. The relationship between subjective preferences and alpha waves on the scalp has been studied by using the ACF (Ando and Chen, 1996, Chen and Ando, 1996, Chen et al., 1997, Mouri et al., 2000, Soeta et al., 2002a). In these studies, the effective duration of the envelope of the normalized ACF, τe, of EEG alpha waves was analyzed. The results showed that the τe value of alpha waves is longer when the subject is presented with preferred conditions. The spread of alpha waves over the scalp has also been studied by using the CCF (Inoye et al., 1983, Nishio and Ando, 1996, Chen, 1997). A propagation of alpha waves from right to left that corresponds to the change in the magnitude of the interaural CCF has been observed (Nishio and Ando, 1996).
We have recently found that the ACF factor, τe, of alpha waves has a strong correspondence with subjective preference for a flickering light (Soeta et al., 2002a, Soeta et al., 2002b). ACF analysis concentrates on the intra-channel correspondences in the time domain. Numerous studies have reported relationships between EEG coherence and mental processes. Coherence analysis concentrates on inter-channel correspondences in the frequency domain. Accordingly, it is logical to assume that subjective preferences for visual stimuli are reflected in both the intra-channel and inter-channel relations between EEG alpha waves in the time domain. These relationships between subjective preference and alpha waves over the scalp were investigated by using the CCF. We chose subjective preference as a primitive response that would lead the individual away from inappropriate environments and toward desirable ones and relate to aesthetics, affect and cognition (Kaplan, 1987). A flickering light varying the period and the mean luminance was used because it is simple and basic.
Section snippets
Subjective preference test
The light source was a 7-mm diameter green LED, set at a distance of 0.6 m from the subject in dark surroundings. The stimulus field from the LED was spatially uniform and its size corresponded to 0.67° of visual angle. Stimulus waveforms were generated by a computer with a 16-bit digital-to-analog converter. The parameters are shown in Fig. 1. The luminance of the stimulus is given bywhere L0 is the mean luminance, m is the relative amplitude of modulation (fixed at 1.0),
Results
The effects of subjective preference and test electrode position on the |φ(τ)|max values were examined for all ten subjects by using two-way ANOVA. The results clearly indicated that |φ(τ)|max values were only significantly affected by subjective preference when period alone was varied and when both period and mean luminance were varied. This is shown in Table 2. When the period was varied, the value of |φ(τ)|max was significantly larger for the most preferred stimulus than for the least
Discussion and conclusions
When the period is varied, the preferred stimulus has a significantly larger value of |φ(τ)|max than that of the least preferred stimulus. The |φ(τ)|max signifies the degree of similar repetitive features that appear in alpha waves recorded at two spatially separated electrodes. Significantly larger values of |φ(τ)|max for alpha waves indicates the brain is repeating a similar rhythm over a wider area under a preferred condition. A number of studies have found that, at certain electrodes, τe
Acknowledgements
This research was supported by a Grant-in-Aid for Science Research from the Japan Society for the Promotion of Science.
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